1. Field of the Invention
This invention relates to digital electrical, position transducers which measure the position of a plurality of parallel decoded channels formed on a mask extending parallel to the direction of motion of a movable member, relative to a bank of detectors extending transversely to the length of the channels, and more particularly to such a system wherein transitions in the binary values of each of the digits of the output occur in synchronism with transitions in the output of the least significant digit.
2. Prior Art
Electrical position transducers which sense the relative position between an elongated code member extending parallel to the direction of motion of a movable member, and a bank of detectors, one for each channel, which extend transversely to the direction of motion, fall broadly into two classes: brush and commutator devices of the type disclosed in U.S. Pat. No. 3,855,708, assigned to the assignee of the present invention, wherein channels represent conductive or non-conductive sections formed on a baseboard; and optical devices employing an elongated mask having channels encoded in the form of transparent and opaque sections and detectors employing light sources and photo-sensors, such as the structure disclosed in U.S. Pat. No. 3,223,845. These devices generate plural digit binary output signals having values representative of the relative position of the code member with respect to the detector. Typically one channel is provided on the code member for each binary digit of the output and a single detector is provided for each channel. The detectors provide a binary output signal having one value when the point on the code channel in their proximity is transparent, or conductive in the case of the commutator type, and another value when the channel section is opaque or nonconductive.
In such a device it is important that the output reading be independent of the direction of motion of the code member relative to the detectors in arriving at a particular position and that the transitions between high and low in the higher order digits of the binary output occur in exact synchronism with the transitions of the least significant digit of the output. In most prior art devices these problems have been treated by improving the precision of the coding in the mask or commutator and the precision of the detectors; however, this approach has definite limits and is highly expensive. Another arrangement has been proposed wherein a pair of detectors are provided for each channel with the exception of the least significant bit channel for which only a single detector is provided. These detector pairs are displaced with respect to one another along the length of a channel and the output signals from all of the detectors are processed with logic which only allows the switching of a particular higher order channel when all of the lower order channels switch. In devices of this sort the spacing between the two detectors that examine the same channel must be related with a high degree of accuracy to the elemental longitudinal dimension of the encoding. Not only is this distance difficult to control, but it requires that the detector set, or at least the spacing between the two members of each channel set, be changed when code plates of different elemental dimensions are employed.